Constitutional issues were an integral part of the controversy over U.S. membership in the League of Nations. Opponents of membership contended that the League would diminish national sovereignty, violate principles of federalism, and interfere with the separation of powers between the president and Congress. In particular, opponents warned that the League would impair congressional and presidential war powers, restrict federal authority over many domestic issues – particularly labor regulations, tariffs, and immigration – and limit the police powers of the states. Proponents of the League contended that constitutional objections were no more than a pretext for political opposition and that Congress, the president, and the U.S. Supreme Court would never interpret the League’s covenant in a manner that brought it into conflict with the Constitution.

This article examines these constitutional issues and considers the extent to which they contributed to the U.S. Senate’s rejection of the League. The article concludes that “irreconcilable” foes of the League were motivated mostly by political objections, but that the so-called “reservationists” raised serious constitutional questions that could have been resolved in favor of membership in League if President Wilson had been willing to compromise with the reservationists.